8-bore;
In spite of its 6 3/4 lb weight this 16 is on an XX frame. I had come to the conclusion the XX denoted a frame with a narrower bar width as this one is narrower than all my others which except for the big 10 have bars of essentially the same width. However I have since heard of a 12 ga having the same narrower bar as this XX 16 which did not have the XX mark.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra